math

Hi I need someone to check my answers I'll show whatever work I have but for time purposes I'll just post my answers because I think I have most of them correct. Thanks!

1)Find the 20th term of the arithmetic sequence in which a1=3 and d=7.
A)143
B)136
C)140
D)133
I chose B

2)Write an equation for the nth term of the arithmetic sequence -3,3,9,15...
A)aN=n+6
B)aN=6n+9
C)aN=6n-9
D)aN=n-3
I chose B

3)Find the two arithmetic means between 4 and 22.
A)10,16
B)8,16
C)8,12
D)13,13
I chose A

4)Simplify: Find Sn for the arithmetic series in which a1=3,d=1/2,and aN=15
A)225
B)9
C)45
D)210
I chose B

5)ok this one's tricky to type but its with sigma so here it is.
There is a sigma sign and there is an 8 on the top of it and k=3 below. In parentheses to the right is (40-3k) and the question is Find.
A)45
B)282
C)-90
D)141
I chose D. this question is tricky but if you read it slowly and you know about sigma you'll be able to help. Thanks again.

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asked by Jon
  1. Your #2 and 4 are not correct

    #2:

    you picked B which is tn = 6n+9
    if you let n=1 you get 15 as your first term, you should obtain -3
    doesn't C give you that result?

    #4:

    tn = 15
    15 = 3 + (n-1)(1/2)
    12 = (n-1)/2
    24 = n-1
    n=25

    So S25 = 25/2[6 + 24(1/2)]
    = 25/2[18]
    = 225 which is A

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    posted by Reiny
  2. ok thanks. Did you check the others?

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    posted by Jon
  3. the others were ok

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    posted by Reiny

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